1. Field of the Invention
The invention is in the field of integrated optics. More particularly, it relates to a polarization filter for cleaning up an optical signal entering via a monomodal input channel and containing both the TE and the TM polarization modes to produce an optical signal emerging via a monomodal output channel and exclusively containing, at least essentially, one of the two polarization modes.
2. Prior Art
Since waveguides are formed at the surface of a substrate in integrated optics, integrated optical components are in general polarization-sensitive. This sensitivity is a consequence of the fact that the polarization for which the dominant electric field component of the optical signal is perpendicular to the surface, i.e. the TM polarization, experiences a different propagation from the polarization for which the dominant electric field component is parallel to the surface, i.e. the TE polarization. Since an optimum design of a component can generally be produced only for one of the polarizations, the presence of the other polarization is therefore troublesome. Polarization filters are known which filter off this troublesome polarization. Thus, reference [1] (see under C.) discloses a filter in which the TM polarization in the optical signal in a waveguide is suppressed with the aid of a metal layer placed above the waveguide. Although this known principle is in fact simple, it has the drawback that it cannot be applied to the suppression of the TE polarization. In addition, the use of metallized elements in an integrated optical design requires additional measures to prevent interfering action on surrounding component parts. References [2] and [3] furthermore disclose TE and TM filters, on GaAs and InP respectively, in which complicated layer packets ("superlattice") are used to achieve selective suppression of one of the two polarizations, it being possible to reinforce the degree of suppression by combination with a bend structure. A drawback of these TE and TM filters is that their fabrication is expensive owing to the complicated structure. In addition, the suppression achieved (14 dB has been reported) is limited.